چەندین ڕێکخراوی سەرەکی کام ئەمێشە بەکاردێت لە هاتووچوونی پاراستنی دیفرانسیەلی نیوەکەی ترانسفۆماتۆری ئەنەرژی؟

Pêşkêşkirina Derivatîvî Longitudinal ya Transformer: Pirsgirênên Sereke û Rewşên Cewabdar

Pêşkêşkirina derivatîvî longitudinal ya transformer yana hêna bikaranînra ji hêmana pêşkêşkirinên derivatîvî yên pirkan. Li ser dema xebitandina, li taybetmendiyekan derdikar dihêne. Li dawên statistîkên sêra 1997 ji rewşeyên 220 kV û piçeyên Transformer ên Navenda Bakhurê, 18 karînên nederbasdigire hatine, ji wan 5 li ser pêşkêşkirina derivatîvî longitudinal bo- îsbar e- her çarba divê. Asûdan maloperasyon û bêtoperasyon dike navbera pirsgirênên operasyon, pêşgotin, û reyîn, û pirsgirênên da ku ji destpêkên çêkirina, nas, û dizayn de derket. Vê gotara pirsgirênên sereka cihan û rêwşên praktîkî yên jêbirde kirin.

1. Girêja Nederbasdigir li ser Transformer Inrush Current

Li dema operasyona normal, girêja magnetîk tenê di rûpelê energîzda de derheqet û girêja nederbasdigir di pêşkêşkirinên derivatîvî de derket. Tenê, girêja magnetîk an jêrî 3%–8% di her çarba divê; ji bo transformeryên mezin, hêj ji 1%. Di dema serokanên dervey de, girêja voltajê derher û girêja magnetîk dikare, girêya çêkerbûn dibêje. Lakin, di dema energîzekirina transformerê bi tevahî û sererastkirina voltajê pas serokanên dervey, girêja inrush mezin dikare- ji 6–8 her çarba divê.

În inrush dike navbera komponentên non-periodik mezin û harmonîkên sêra, tenê harmonîka duyem, û wisaforma girêja necontinuous (dead angles).

Rêwşên jêbirdekirina li ser pêşkêşkirina derivatîvî longitudinal:

(1) Relays BCH-type bi transformers current fast-saturating:
Di dema serokanên dervey, komponenta non-periodik mezin core di transformer fast-saturating de derher dikare, girêja nederbasdigir ji ber hilka relay coil- û jêbirdekirina false tripping. Di dema serokanên internal, ewer komponentên non-periodik di çavkanîn, lakin bi ~2 cycles derher dikare. Pasan, tenê girêja serokan periodik derheqet, û relay sensitive derheqet.

(2) Relays microprocessor-based bi second-harmonic restraint:
Ji bo zêdeyi relays digital modern, second-harmonic blocking bikar anîn ji bo serbestkirina inrush ji serokanên internal. Ji bo misoperation di dema serokanên dervey sererastkirina:

• Bigrada ji mode "AND" ("AND") restraint ji phase-by-phase bi mode "OR" ("OR") restraint.

• Kêm bike second-harmonic restraint ratio ji 10%–12%.

• Li sisteman mezin kapasite ku fifth-harmonic content ji bo sererastkirina serokan de ye, zêde bike fifth-harmonic restraint.

• Ji bo transformeryên ku dual differential protections hene, biceribike principles symmetry waveform ji bo identifikasyon inrush- ev rêwsha jêrî sensitive û reliable yekîsh.

2. Wiring Nederbasdigir li ser CT Secondary Circuits

Yek pirsgirêk recurring maloperasyon yana polarity reversed di terminalên secondary current transformer (CT) de- ji destpêkên pêşgotin nehatî, devasyon ji vêneyên dizayn, û pêşgotin testing commissioning.

Praktîk preventive:
Pêve pêşkêşkirina derivatîvî longitudinal di ser dema new installation, testing periodic, û her vegerîn di circuit secondary, transformer must be loaded, û checks following performed:

• Measure the unbalanced voltage in the differential loop using a high-impedance voltmeter; it must comply with code limits.

• Measure magnitude and phase angle of secondary currents on all sides.

• Construct a hexagonal vector diagram to verify that the vector sum of same-phase currents is zero or near-zero, confirming correct wiring.

Only after these verifications should the protection be formally commissioned.

3. Poor Contact or Open Circuit in Secondary Circuits

Misoperations due to loose connections or open circuits in CT secondary loops occur annually.

Recommendations:

• Strengthen real-time monitoring of differential current during operation.

• After relay installation/commissioning or major transformer overhauls, inspect all CT secondary connections.

• Tighten terminal screws and use spring washers or anti-vibration clips.

• For critical applications, use two parallel cables for the differential secondary wiring to mitigate open-circuit risk.

4. Grounding Issues in Differential Protection Secondary Circuits

Some sites violate anti-accident measures by having two grounding points—one in the protection cabinet and another in the switchyard terminal box. The resulting ground potential difference, especially during lightning or nearby welding, can induce spurious differential current and cause false tripping.

Solution:
Strictly enforce single-point grounding. The only reliable ground point should be located inside the protection cabinet.

5. Insulation Degradation of CT Secondary Cables

Insulation failure in CT secondary cables—often due to poor construction practices—also leads to misoperations. Common causes include:

• Cable sheath damage during laying,

• Splicing two cables when length is insufficient,

• Welding cable conduits with cables inside, causing thermal damage.

These create hidden risks to protection reliability.

Preventive measures:

• During major equipment maintenance, periodically test insulation resistance between each core-to-ground and core-to-core using a 1000 V megohmmeter; values must meet code requirements.

• Keep exposed wire ends at terminals as short as possible to prevent accidental grounding or phase-to-phase short circuits due to vibration.

6. Selection of Current Transformers for Longitudinal Differential Protection

Differential protection involves CTs across different voltage levels, with varying ratios and models, leading to mismatched transient characteristics—a potential source of misoperation or failure to operate.

• 500 kV side: Use TP-class CTs (transient-performance class), whose gapped cores limit remanence to <10% of saturation flux, greatly improving transient response.

• 220 kV and below: Typically use P-class CTs, which have no air gap, higher remanence, and poorer transient performance.

Selection guidance: While TP-class CTs offer superior technical performance, they are expensive and bulky—especially on the low-voltage side, where installation in enclosed bus ducts is difficult. Therefore, unless special system requirements exist, P-class CTs should be preferred if they satisfy actual operational needs—avoiding unnecessary cost and installation challenges.

Additionally, secondary cable cross-section must be adequate:

• For long cable runs, use ≥4 mm² conductor size to minimize burden and ensure accuracy.